This invention relates to X-ray imaging systems, and, in particular, to X-ray imaging systems employing optic devices to produce collimated, fan-shaped beams having desired spectral shape.
Imaging applications, such as CT and X-ray diffraction, require ever-increasing levels of flux. Increasing X-ray flux may be accomplished, for example, by focusing X-ray radiation emitted by an X-ray source. X rays may be focused by reflecting an incident X-ray beam from an interface using total internal reflection. The interface can be formed between a first material medium having nf as a complex refractive index, and a second material medium having ns as a complex refractive index. Typically, the first material medium may be air, and the second material medium may be a solid. Total internal reflection can be realized if the real part of the complex refractive index ns of the second medium is smaller than the real part of the complex refractive index nf of the first medium, and if the angle of incidence of the X ray with the interface is smaller than a critical angle θCR specified for total internal reflection. However, the conventional method of selecting materials for the first material medium, and second material medium, solely on the basis of the material indices of refraction produces only modest gains in reflectivity.
In addition to increasing flux levels, spectral shaping an X-ray spectrum is another requirement for optimizing the X-ray spectrum for particular applications. One common artifact in radiographic and tomographic imaging arises from the fact that the lower energy X rays in a typical Bremsstrahlung (polychromatic) spectrum are attenuated preferentially as the beam penetrates material. This effect, which leads to an increase in the mean energy of the beam as it penetrates the sample, introduces a non-linearity in the relationship between the strength of the transmitted beam and the amount of material penetrated. This non-linearity manifests as artifacts in images reconstructed from the attenuation data, such as those attributed to beam hardening in computed tomography. Utilizing an X-ray beam that has a reduced spread of energies can mitigate some of these artifacts. Particularly where beam intensity, with respect to the intensity in that same range of the original spectrum, has been held constant or augmented by the use of the optic, the use of a limited range of energies can provide a desired degree of attenuation for a particular application and can produce an optimum image in terms of spatial resolution and contrast sensitivity. The shaping of a spectrum from a polychromatic energy distribution to a more monochromatic distribution enables such improvements in X-ray image sets. In some cases, change in the spectral shape, for example, reducing either the relative proportion of low-energy or high-energy X rays, may provide for optimum imaging of a sample.
However, multi-energy X-ray imaging, sometimes referred to as dual-energy imaging or energy discrimination imaging, has its own benefits. For example, multi-energy X-ray imaging has been shown to furnish information on specific material compositions in scanned objects for security, industrial, and medical applications. Such energy discrimination imaging can be achieved in several ways, including the use of two or more different X-ray spectra, which is often the most feasible approach. A challenge lies in the sequential nature of such an examination, where image data are generated, for example, first with one spectrum and then with another spectrum. In one technique, an object of interest is scanned twice. A first complete projection data set is produced in the first scan for one energy and then a second complete projection data set is produced in the second scan for the second energy. For many applications where high throughput is critical, sample composition is dynamic, and/or sample positioning may preclude repetitive scanning, the logistics of physically scanning an object twice may be unacceptable.
Conventional multi-energy X-ray imaging applications have used source filtration and/or high voltage modulation for rapidly altering the spectral characteristics on a time scale comparable to the view-by-view sampling time in a typical imaging scan. Such filtration consists of rapidly and sequentially inserting filters of appropriate composition to preferentially attenuate relatively low X-ray energies. Such methodologies are limited in the degree to which attenuation can produce cleanly separated energy intervals, severely restricting the sensitivity of this approach for analyzing different materials. High voltage modulation to produce different spectral characteristics also has been implemented in some cases with limited success. There is a challenge in both approaches to mitigate registration differences in the projections that result from sample movement between data sets acquired at different energies, as well as a slight misalignment of the X-ray paths that traverse the object, as is incurred with modulating the X-ray beam on a sub-view basis.
Typically, fan-shaped beams are used in a wide variety of polychromatic X-ray imaging situations to furnish information on specific material composition in scanned objects for security, industrial, and medical applications. For example, fan-shaped X-ray beams are used in X-ray imaging, such as for mammographic and general radiographic imaging in the medical field; computed tomography imaging; tomosynthesis imaging; and X-ray diffraction imaging.
Most conventional X-ray sources have a single X-ray generation spot whose effective size and location are determined and/or limited by the anode thermal loading and relative angle of emission. The X-ray spot is typically collimated using tungsten or lead in the transaxial (fan angle) and axial (cone angle) imaging directions. To use 2D reconstruction algorithms for CT (i.e., filtered back-projection), the X-ray spot is collimated such that only thin, pseudo-planar, fan-shaped sheets of X rays illuminate the imaged object. As a result, only a small percentage of the available X-ray photons from the spot are used for imaging as most X rays strike the collimator plate and are absorbed. To utilize a higher percentage of the available X rays, the cone angle of the X-ray beam can be broadened; however, more complicated cone-beam reconstruction algorithms are needed to achieve acceptable image quality for targeted applications. Hence, there is a tradeoff of X-ray flux utilization versus reconstruction complexity that exists in any CT imaging systems.
It would thus be desirable to have a reflective multilayer configuration that can provide fan-shaped X-ray beams having a desired spectral shape of the X-ray beam for X-ray imaging applications.